The present invention pertains to an evaporative emission control system for a vehicle. More particularly, the invention is directed to a system with a vent valve installed in a vent from the evaporative emission control system storage canister for use in system diagnosis.
Automobiles currently include a system for the collection and processing of fuel evaporate created by fuel carried in the fuel system of the automobile, primarily the storage tank. The type of evaporative emission control system involved in the present invention is the evaporative emission canister storage method. This method is comprised of a mechanism to transfer fuel vapor and air emanating from the fuel tank to a canister typically containing activated carbon as an adsorbent for adsorption, storage and later recovery of the vapor.
Adsorption is a reversible process in which gas or liquid particles adhere in a thin film to the surface area of the solid adsorbent. The fuel vapor that is collected in the storage canister is held by the adsorbent when the vehicle is not operating and under certain operating conditions. When the vehicle's engine is running and selected operating criteria are met the vapors are purged from the adsorbent. During the purge cycle, the stored fuel vapor is stripped from the adsorbent by passing ambient air through the adsorbent bed and then mixing it with the intake air flow of the engine for consumption in the normal combustion process. The adsorbent is thereby regenerated by the purge cycle.
The adsorption-desorption process is dynamic and ongoing with the cyclic operation and parking of an automobile that results from a typical operator's use of the vehicle for transportation. Vapors emitted during periods the vehicle is parked are adsorbed and then later desorbed when the vehicle is driven. It is important to provide a free flow of air through the canister during the purge cycle to thoroughly strip the stored vapor from the adsorbent. This results in thorough desorption while the vehicle is in operation thereby providing adsorption capacity for later parked periods.
The storage canister of the evaporative emission control system generally has three port openings leading to the adsorbent bed. A first opening is connected to a line that allows air and fuel vapor to pass from or to the vehicle's fuel tank according to pressure gradients in the tank. A second opening is connected to a line leading to the intake manifold of the vehicle's engine allowing purged vapor and air to pass to the engine according to selected engine operating conditions. The third opening is typically vented to the atmosphere allowing the canister to breathe freely.
By permitting air flow between the canister and atmosphere, the vent facilitates evaporate transfer between the canister and fuel tank and supplies air for canister purge. The vent permits fuel tank evaporate to enter the canister during periods of rising pressure within the tank by acting as a pressure relief point for the system. Evaporate may also return to the tank from the canister during periods of falling pressure in the tank, with the vent relieving vacuum in the system by providing an air entry point. Additionally, during a purge cycle air is drawn through the vent into the adsorbent bed of the canister stripping evaporate and carrying it to the engine for consumption. The vent to the atmosphere is therefore, an essential component of the evaporative emission control system.
The system, generally as described, was introduced to use in automobiles to reduce hydrocarbon emissions from the fuel system of vehicles and is in widespread use in automobiles today. The most common current mechanism for diagnosing existing evaporative emission control systems is through manual inspection and testing of a system. However, technology has progressed to the point that efforts have been made to provide an automatic mechanism to diagnose systems. Attempts have been made to close systems to a greater extent and provide an automatic diagnostic means. Automatic systems known in the art generally include a valve in the evaporative emission control system's canister vent that is coupled to the vehicle's electronic control module or electrical system.